1. Field of the Invention
This invention relates to an abrasive article having a sheet-like structure having deployed thereon a plurality of individual abrasive composites, each of which is comprised of a plurality of abrasive particles dispersed in a plasticized binder. The invention also relates to a process of making such as abrasive article and a method of using such an abrasive article to reduce a workpiece surface to impart a relatively fine surface on the workpiece being graded.
2. Discussion of the Art
In general, abrasive articles comprise a plurality of abrasive particles bonded either together (e.g., a bonded abrasive or grinding wheel) or to a backing (e.g., a coated abrasive). These abrasive articles have been utilized to abrade and finish workpieces for well over a hundred years. Within the last several years, abrasive technology has grown to include structured abrasives. These abrasive articles are desirable because of their long life and high rate of stock removal. It has been seen that a structured abrasive can remove more stock than an abrasive coating employing the same size of abrasive particles.
Coated abrasives and structured abrasives can be employed for polishing operations, i.e., providing a very fine surface finish on the workpiece. However, when an ultrafine surface finish is desired, such as that required for an optical lens which require a clear surface finish, loose abrasive slurries are typically used.
However, the use of loose abrasive slurries for polishing and ultrafine finishing has drawbacks. For instance, the use of a loose abrasive slurry can be rather untidy as the extraneous slurry is thrown about in the work area by the motion of the lapping or polishing wheel or block. Also, the use of loose abrasive slurries can be less cost efficient as it may be difficult to estimate up front the minimal amount of needed abrasive material. This leads to the use of excessive amounts of abrasive, equipment and manpower. The industry has sought means to generate an ultrafine surface finish without the need to use a loose abrasive slurry.
A method for polishing using a solid abrasive polishing material has been proposed as a substitute for a loose abrasive slurry, such as disclosed in U.S. Pat. No. 3,042,509 to Soderburg. The abrasive material is constituted by a dispersion of abrasive particles in a binder where the binder is based on a water-soluble binder such as polyethylene glycol ester. Polyethylene glycol is blended with this water-soluble binder to provide a solid substance that is exemplified as formable into a stick-form that is urged against and applied to the outer surface of a buffing wheel.
To provide a hard and durable abrasive composite it has been proposed to form a mixture of abrasive particles and a temporary binder type material, such as polyethylene glycol, into a desired shape to obtain a green body as an intermediate product. The green body is sintered at high temperature to densify the abrasive body into a useful form that, concomitantly, acts to decompose and remove the temporary binder. For example, see U.S. Pat. Nos. 4,918,874 to Tiefenbach, Jr.; 3,765,300 to Taylor et al.; and 4,035,162 to Brothers et al.
The possible inclusion of plasticizers as an optional additive to an abrasive slurry based on a nonwater-soluble thermoset or reactively-cured binder in forming structured abrasive composites has been generally suggested without elaboration in several recent patents, such as U.S. Pat. Nos. 5,152,179 to Pieper et al. and 5,219,462 to Bruxvoort et al. Further, the use of a binder system in a structured abrasive composite that employs a binder polymerized via a free radical mechanism has been shown, such as in U.S. Pat. No. 5,152,179 to Pieper et al.
Also, U.S. patent application Ser. No. 08/030,787 (Christianson), filed Mar. 12, 1993, teaches a stone polishing abrasive article comprising radiation curable resin in a three-dimensional dot pattern. An amount of plasticizer, such as polyethylene glycol, of less than 30% based on weight of plasticizer and binder is mentioned as an additive for a binder, while the working examples describe usage of about 6 to 10% plasticizer. Additionally, the use of relatively small amounts of plasticizers such as polyethylene glycol, that is less than 10% by weight based on the weight of binder and plasticizer, in microfinishing beads or agglomerates also has been practiced to cause the beads to wear during a grinding process to expose new sharp mineral surfaces.
U.S. Pat. No. 4,255,164 to Butzke et al. disclose a glass fining sheet composed of a foamed liquid abrasive granule-resin coating composition. The resin is a cured modified resinous binder selected from urea-formaldehyde and phenol formaldehyde that has been modified by a thermoplastic polymeric modifier. The liquid coating composition comprises the liquid curable binder, abrasive .fining granules and sufficient compatible solvent to provide a coatable composition. Such a coating provides a cellular layer which releases the fining abrasive granules at a controlled rate under use conditions. Butzke et al. also describe prior use of means to incorporate fining abrasive material into a cohesive layer so as to release abrasive material during glass grinding, but these means not having met with success. Prior attempts are also mentioned by Butzke et al. to cause the binder to disintegrate, dissolve or soften to free abrasive granules, such as by adding lubricants such as stearic acid, tallow, and paraffin wax. However, these prior attempts are described as unsatisfactory as the binder material disintegrates too rapidly and problems arose with respect to unmanageable frictional heat generation.
It has also been generally known to add polyalkylene oxides to resins that do not cure via a free radical mechanism, such a condensation curable resins such as phenolic resins. For instance, U.S. Pat. No. 4,576,612 to Shukla et al. describe an ophthalmic lens polishing pad where the polishing layer is produced by mixing a water soluble polyalkylene oxide/phenolic resin complex with an acrylic latex, and an alcohol slurry containing polishing particles. Shukla et al. state that the use of a water soluble polymer (polyalkylene oxide/phenolic resin mixture) exclusive of latex released polishing particles too rapidly with consequent poor polishing results. The polishing layer in Shukla et al. is provided as a continuous monolithic layer on a fabric substrate, or, alternatively, as a layer to completely cover or partially fill recesses in an embossed surface of the fabric substrate. The so-called thermoplastic matrix or binder system gradually dissolves during polishing to release polishing particles in a controlled manner to thus reportedly provide an acceptable glass removal rate.
However, while the use of such water soluble thermoplastic resin binder systems may be acceptable for simple abrasive coating layers or modified abrasive coating layers (e.g., embossed), the requirements for and demands placed upon the binder system generally will become more rigorous if a coated abrasive article is based on a more sophisticated arrangement, such as the deployment of individual abrasive beads or shaped abrasive composites upon the surface of a backing. The requirements there are heightened from the standpoint of manufacturing consistency, ease and rate, and from the standpoint of degree of control afforded over the ultimate shapes of the individual abrasive composites, which can be a critical design aspect. Also, the use of condensation curable resins, such as phenolic resins, in the binder system may not be tolerable in all cases in view of solvent emission considerations.
On the other hand, the provision of relatively large amounts of plasticizer in a binder that is cured via a free radical polymerization mechanism to provide an acceptable, if not desirable, erodable abrasive composite during finishing operations is not thought to have been taught before.